Method for encoding and decoding moving picture signals

BACKGROUND OF THE INVENTION

0001Notice: More than one reissue application has been filed for the reissue of U.S. Pat. No. <b>5</b>,<b>937</b>,<b>095</b>. The reissues applications are application Nos. <b>10</b>/<b>662</b>,<b>949</b> which is a continuation of <b>09</b>/<b>925</b>,<b>423</b> (<i>the present application</i>).

0002(1) Field of the Invention

0003The present invention relates to a method for encoding and decoding digital moving picture signals for use in TV phones, TV conferences and the like.

0004(2) Description of the Prior Art

0005In a general method for encoding digital moving picture signals, a frame of inputted moving picture is divided into plural blocks each composed of N×M pixels, and processes of motion detection, prediction, orthogonal transform, quantization, variable length coding, etc. are conducted on each block.

0006In a general method for decoding digital motion picture signals, blocks each composed of N×M pixels are regenerated in a reverse procdyreprocedure, that is, processes of variable length decoding, reverse quantization, reverse orthogonal transform, motion compensation, etc.

0007The above general encoding method and decoding method for encoding and decoding digital moving picture signals enable removal of redundancy contained in moving picture signals, and efficient communication and storage of a moving picture with less information.

0008In the general encoding method and decoding method for encoding and decoding digital moving picture signals, the processes are conducted on each pixel block, as stated above. It is general that a set of pixel blocks forms a subframe and a set of subframes forms a frame, which are units processed in the general encoding and decoding method.

0009Hereinafter, encoding and decoding of each block, subframe and frame will be described by way of an example of a general encoding and decoding method for encoding and decoding digital moving picture signals with reference to ITU-T Recommendation H.261 (hereinafter, referred simply H.261) made on March, 1993.

0010H.261 defines an encoding method and a decoding method for encoding and decoding luminance signals and color difference signals, separately, of digital moving picture signals. However, description will be made of only the luminance signals, for the sake of convenience. Basically, the encoding method and decoding method for encoding and decoding the luminance signals are not different from those for the color difference signals.

0011As shown in <figref idref="DRAWINGS">FIG. 1</figref>, one frame <b>101</b> of digital moving picture signals is composed of 352×288 pixels according to H.261. The frame <b>101</b> is divided into twelve subframes <b>102</b> called GOBs (Group of Blocks) each composed of 176×48 pixels (hereinafter, the subframe in the description of the prior art will be referred a GOB). Further, the GOB <b>102</b> (subframe) is divided into thirty three blocks <b>103</b> called macro blocks each composed of 16×16 pixels.

0012The encoding method according to H.261 defines that encoded information for one frame is corresponded to a spatial hierarchical structure such as the frame <b>101</b>, GOBs <b>102</b> and macro blocks <b>103</b> described above, as shown in FIG. <b>2</b>.

0013In <figref idref="DRAWINGS">FIG. 2</figref>, a part enclosed in a rectangle shows encoded information, and the number of coding bits is shown under each of the rectangles. In <figref idref="DRAWINGS">FIG. 2</figref>, arrows show linkages of the encoded information. A series of encoded moving picture signal sequences as this is called a bit stream <b>104</b>.

0014In the bit stream <b>104</b> according to H.261 shown in <figref idref="DRAWINGS">FIG. 2</figref>, a part including all encoded information for one macro block <b>103</b> is called a macro block layer <b>103</b>S, a part including all encoded information for once GOB <b>102</b> is called a GOB layer <b>102</b>S, and a part including all encoded information for one frame <b>101</b> is called a frame layer <b>101</b>S.

0015Meanings of the encoded information in each of the layers shown in <figref idref="DRAWINGS">FIG. 2</figref> are given below:

0000Frame Layer <b>101</b>S

0016PSC (20 bits): a frame identifier <b>105</b>; a unique code by which an encoding method can be always identified, expressed as “0000 0000 0000 0000 0001”;

0017TR (5 bits): a frame number <b>106</b>; indicating a time position in which this frame <b>101</b> should be displayed;

0018PTYPE (6 bits): frame type information <b>107</b>; various information about the frame <b>101</b>;

0019PEI (1 bit): extension data insertion information <b>108</b>; a flag representing presence of following PSPARE <b>109</b>;

0020PSPARE (8 bits): extension data; GOB layer <b>102</b>S (subframe)

0021GBSC (16 bits): a GOB identifier <b>110</b>; a unique code by which a decoding method can be always identified, expressed as “0000 0000 0000 0000”;

0022GN (4 bits): a GOB number <b>111</b>; indicating a spatial position of this GOB <b>102</b> within the frame <b>101</b>;

0023GQUNAT (5 bits): quantization characteristics information <b>112</b>; indicating a quantization characteristic when a macro block <b>103</b> in the GOB <b>102</b> is encoded;

0024GEI (1 bit): extension data insertion information <b>113</b>; a flag representing presence of following GSPARE <b>114</b>;

0025GSPARE (8 bits): extension data <b>114</b>.

0026Incidentally, the encoded information <b>115</b> of the macro block layer which is the lowest hierarchy in <figref idref="DRAWINGS">FIG. 2</figref> is generated in the encoding method of motion detection, prediction, orthogonal transform, quantization, variable length coding, etc., as described before, whose coding bit number is not fixed. The number of coding bits of the macro block layer <b>103</b>S, in general, increases if a spatial level of pixels included in the macro block <b>103</b> changes largely or a time level of pixels included in the macro block <b>103</b> having the same spatial positions changes largely. Such macro block <b>103</b>, is hereinafter, referred a macro block <b>103</b> which is difficult to be encoded.

0027To the contrary, if a level of pixels included in the macro block <b>103</b> is steady in relation to space and time, the number of coding bits of the macro block layer <b>103</b>S remarkably decreases, or sometimes becomes zero. Such macro block <b>103</b> is hereinafter referred a macro block <b>103</b> which is easy to be encoded.

0028In the decoding method according to H.261, the PSC <b>105</b> which is an identifier of the frame layer <b>101</b>S is first found out from the bit stream <b>104</b>. Incidentally, in a state where a decodable code has been successfully found out it is said that synchronization is established. When the PSC <b>105</b> is found out from the bit stream and synchronization of the frame layer <b>101</b>S is established, it can be identified that the bit stream <b>104</b> until the next PCS <b>105</b> appears is encoded information for one frame. Further, a time position in which the frame <b>101</b> composed of 352×288 pixels obtained by decoding the bit stream <b>104</b> for that one frame can be obtained by examining the frame number <b>106</b> following the PSC <b>105</b>.

0029After the establishment of the frame layer, a GBSC <b>110</b> that is an identifier of the GOB layer <b>102</b>S is found out from the following bit stream <b>104</b> in the encoding method according to H.261. When synchronization of the GBSC layer is established, it can be identified that the bit stream <b>104</b> until the next GBSC <b>110</b> appears is encoded information for one GOB <b>102</b>. Further, a spatial position of the GOB <b>102</b> composed of 176×48 pixels obtained by decoding the bit stream <b>104</b> for that one GOB <b>102</b> in a frame <b>101</b>, in which the GOB <b>102</b> should be placed, can be obtained by examining a GN <b>111</b> which is a GOB number following the GBSC <b>110</b>.

0030In the decoding method according to H.261, a bit stream <b>104</b> of a following macro block layer <b>103</b>S is decoded after the establishment of the GOB layer <b>102</b>s. The decoding method of the macro block layer <b>103</b>S is a procedure to regenerate a macro block <b>103</b> composed of 16×16 pixels in processes of variable length decoding, reverse quantization, reverse orthogonal transform, motion compensation, etc., as described before. It should be here noted that the macro block layer <b>103</b>S has no unique code by which a decoding method can be always identified dissimilarly to the PSC <b>105</b> or BGSC <b>110</b>, and encoded information of each macro block is composed of undefined length bits of a variable length code.

0031As shown in <figref idref="DRAWINGS">FIG. 3</figref>, in the GOB (subframe) layer <b>102</b>S, the encoded information from the first macro block <b>115</b>, to the thirty third macro block <b>115</b>₃₃is expressed as a series of variable length codes without a unique code. If decoding of the macro block encoded information is initiated from a point indicated by A in <figref idref="DRAWINGS">FIG. 3</figref>, and successively conducted in the order of the first, the second, . . . the nth, . . . the thirty third macro blocks, it is possible to regenerate all the macro blocks <b>103</b> in the GOB layer <b>102</b>S. However, if the decoding of the macro block encoded information is initiated from a point indicated by B or C in <figref idref="DRAWINGS">FIG. 3</figref>, it is impossible to identify a point from which encoded information <b>115</b> of one macro block starts, which leads to a failure of establishing synchronization. In which case, the decoding and regenerating all macro blocks <b>103</b> become unfeasible until the next GBSC <b>110</b> appears. In other words, the GBSC <b>110</b> also represents a starting point of decoding the macro block layer <b>103</b>S.

0032Finally, in the decoding method according to H.261, the GOB <b>102</b> which is a set of regenerated macro blocks <b>103</b> is placed in a spatial position within a frame <b>101</b> directed by GN <b>111</b>, and the frame <b>100</b> which is a set of the regenerated GOBs <b>102</b> is placed in a time position directed by TR <b>106</b>.

0033As above, it is possible to decode one frame <b>101</b> of digital moving picture correctly in relation to space and time according to H.261.

0034However, the above general method for encoding and decoding digital moving picture signals has a drawback that if a part of a bit stream <b>104</b>lacksis lacking or an error occurs therein, it might be impossible to accurately decode all subframes (GOBs) <b>102</b> in relation to time until synchronization of the next frame layer <b>101</b>S is established.

0035The reason of the above is that codes which can be identified at all times in the bit stream <b>104</b> are only the PSC <b>105</b> which is a frame identifier and the GBSC <b>110</b> which is a subframe identifier in the general decoding method. If a part of the bit stream <b>104</b> lacks or an error occurs therein, it is impossible to recover synchronization of the decoding until the next GBSC <b>110</b> appears so that the decoding becomes unfeasible. Even if the next GBSC <b>110</b> appears, the bit stream <b>104</b> of that subframe layer <b>102</b>S cannot be correctly decoded in relation to time. This will be understood from FIG. <b>4</b>.

0036<figref idref="DRAWINGS">FIG. 4</figref> shows an example where the fifth GOB <b>102</b>₅in the nth frame <b>110</b>n through the sixth GOB <b>102</b>₆in the (n+1)th frame <b>101</b>_n−1<b>101</b>_n+1cannot be decoded in relation to time due to lackslacking portions or errors of the bit stream <b>104</b> occurring in burst. In this example, not only the PSC <b>105</b> corresponding to the (n+1)th frame in relation to time but also the following TR <b>106</b> are missed or in error. It is therefore possible to correctly decode the GOB <b>102</b>₇in relation to space by establishing synchronization from the GBSC <b>110</b> corresponding to the seventh GOB <b>102</b>₇in the (n+1)th frame <b>101</b>_n+1in relation to time and decoding the following GN <b>111</b>, but impossible to specify whether this GOB <b>102</b>₇positions in the nth frame or in the (n+1)th frame in relation to time.

0037In terms of decoding of the eighth GOB <b>102</b>₈through the twelfth GOB <b>102</b>₁₂in the (n+1)th frame in relation to time, it is impossible to specify whether these GOBs <b>102</b> position in the nth frame or in the (n+1)th frame in relation to time.

0038In consequence, if a part of the bit stream <b>104</b> is missedmissing or an error occurs therein, it becomes impossible to correctly decode all GOBs <b>102</b> in relation to time until synchronization of the next frame layer <b>101</b>₅is established.

0039Further, the general method for encoding and decoding digital moving picture signals has another drawback that if the GOB <b>102</b> including a picture in motion in relation to time cannot be decoded, a picture quality of the reproduced picture is largely degraded.

0040This problem will be described in more detail with reference to FIG. <b>5</b>. <figref idref="DRAWINGS">FIG. 5</figref> shows one frame including decoded signals of a moving picture, where a figure is moving in the center of the frame. In <figref idref="DRAWINGS">FIG. 5</figref>, a part moving in relation to time is indicated by slanting lines, and the remaining part is a background which is still in relation to time. A scene like this is general in TV conferences, TV telephones or the like.

0041Referring to <figref idref="DRAWINGS">FIG. 5</figref>, considering that any one of the first GOB <b>102</b>₁through the fourth GOB <b>102</b>₄cannot be decoded. The first through fourth GOBs <b>102</b>₁through <b>102</b>₄include a picture still in relation to time. If the second GOB <b>102</b>₂cannot be decoded, for example, a skillful operation is conducted to substitute the second GOB <b>102</b>₂of the present frame <b>101</b> with the second GOB <b>102</b>₂of the preceding frame <b>101</b>₋₁in the decoding. With this operation, degradation of a picture quality in the second GOB <b>102</b>₂of the present frame <b>101</b> may be hardly detected.

0042However, it is a problem if decoding of the fifth through twelfth GOBs <b>102</b>₅through <b>102</b>₁₂shown in <figref idref="DRAWINGS">FIG. 5</figref> cannot be decoded. The fifth through twelfth GOSsGOBs <b>102</b>₅through <b>102</b>₁₂include a picture moving in relation to time. This means, for example, that a picture in the ninth GOB <b>102</b>₉of the preceding frame <b>101</b>₋₁is largely different from the ninth GOB <b>102</b>₉of the present frame <b>101</b> in relation to time. If the decoding of the ninth GOB <b>102</b>₉is unfeasible, degradation of the picture quality of the ninth GOB <b>102</b>₉of the present frame <b>101</b> is obviously detected even if the skillful operation mentioned above is conducted in the decoding.

0043Accordingly, if decoding of GOB <b>102</b> including a picture moving in relation to time becomes unfeasible, a quality of a reproduced picture is largely degraded.

SUMMARY OF THE INVENTION

0044In the light of the above problems, an object of the present invention is to provide a method for encoding and decoding digital moving picture signals, which can appropriately decode subframes (GOBs) following a subframe in trouble in relation to time if a part of a bit stream is missing or an error occurs in the bit stream.

0045Another object of the present invention is to provide a method for encoding and decoding digital moving picture signals, which can suppress degradation of a reproduced picture to a small extent if decoding of a subframe (GOB) including a picture in motion in relation to time becomes unfeasible.

0046To accomplish the first object, the present invention is featured in that in the method for encoding and decoding digital moving picture signals of this invention, time position information representing an order of displaying a subframe is attached to an identifier of the subframe by which the subframe is identified.

0047According to the method for encoding and decoding digital moving picture signals of this invention, time position information representing an order of displaying a subframe is attached to an identifier used to identify the subframe and the identifier of the subframe is encoded. It is therefore possible to decode subframes following a subframe in trouble appropriately in relation to time if a part of a bit stream is missing or an error occurs in the bit stream by using the time position information representing an order of displaying each of the subframes attached to an identifier used to identify the subframe

0048To accomplish the second object, the present invention is featured in that in the method for encoding and decoding digital moving picture signals of this invention, the number of blocks included in a subframe is varied according to a sum of quantities of generated information of the blocks included in the subframe so that each of all the subframes included in the frame has an equal sum of quantities of the generated information of the blocks included in the subframe.

0049According to the method for encoding and decoding digital moving picture signals of this invention, the number of blocks included in a subframe is varied according to a sum of quantities of generated information of the blocks included in the subframe so that each of all the subframes included in the frame has an equal sum of quantities of the generated information of the blocks included in the subframe. In consequence, a spatial size of each subframe is not fixed. A subframe including a block having a large number of coding bits is in a smaller size, whereas a subframe including a block having a small number of coding bits is in a larger size. It is therefore possible to suppress degradation of a reproduced picture even if decoding of a subframe becomes unfeasible since a subframe including a block which includes a motion in relation to time and is difficult to be encoded is in a smaller size in relation to space.

BRIEF DESCRIPTION OF THE DRAWINGS

0050<figref idref="DRAWINGS">FIG. 1</figref> shows units to be encoded in a general encoding method for encoding moving picture signals;

0051<figref idref="DRAWINGS">FIG. 2</figref> shows a bit stream generated in the general encoding method for encoding moving picture signals;

0052<figref idref="DRAWINGS">FIG. 3</figref> shows a GOB layer in the bit stream in <figref idref="DRAWINGS">FIG. 2</figref> generated in the general encoding method for encoding moving picture signals;

0053<figref idref="DRAWINGS">FIG. 4</figref> illustrates an effect of a lack or an error of a part of a bit stream occurring in the general encoding and decoding method for encoding and decoding moving picture signals;

0054<figref idref="DRAWINGS">FIG. 5</figref> illustrates an effect of a lack or an error of a part of a bit stream occurring in the general encoding and decoding method for encoding and decoding moving picture signals;

0055<figref idref="DRAWINGS">FIG. 6</figref> shows a bit stream generated in a method for encoding digital moving picture signals according to first and second embodiments of this invention;

0056<figref idref="DRAWINGS">FIG. 7</figref> is a flowchart illustrating the method for decoding digital moving picture signals according to the first embodiment of this invention;

0057<figref idref="DRAWINGS">FIG. 8</figref> illustrates the method for encoding digital moving picture signals according to the second embodiment of this invention; and

0058<figref idref="DRAWINGS">FIG. 9</figref> shows a structure of subframes according to the second embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0059Hereinafter, description will be made of embodiments according to the present invention referring to the drawings.

0060A method for encoding and decoding digital moving picture signals according to a first embodiment will be now described, which may correctly decode a subframe as a unit in relation to time even if a part of a bit stream is missing or an error occurs therein.

0061In the encoding method according to this embodiment, one frame of digital motion picture signals is composed of, for example, 352×288 pixels. The frame is divided into twelve subframes each composed of, for example, 176×48 pixels. Further, the subframe is divided into thirty three blocks <b>13</b> each composed of, for example, 16×16 pixels.

0062The encoding method according to this embodiment corresponds encoded information for one frame to a spatial hierarchical structure made up of a frame <b>11</b>, subframes <b>12</b> and blocks <b>13</b> to generate a bit stream <b>14</b> as shown, for example, in FIG. <b>6</b>.

0063Meanings of encoded information of each layer shown in <figref idref="DRAWINGS">FIG. 6</figref> are given below: Frame layer <b>11</b>S

0064FSC (20 bits): a frame identifier <b>15</b>; a unique code by which a decoding method can be always identified, expressed as “0000 0000 0000 0001 0000”;

0065Subframe Layer <b>12</b>S

0066SFSC (16 bits): a subframe identifier <b>16</b>; a unique code by which a decoding method can be always identified, expressed as “0000 0000 0000 0001”;

0067SFNT (5 bits): a subframe time number <b>17</b>; indicating a time position in which this subframe <b>12</b> should be displayed;

0068SFNS (4 bits): a subframe space number <b>18</b>; indicating a spatial position in which the subframe <b>12</b> should be displayed;

0069SFQUANT (5 bits): quantization characteristic information <b>19</b>; representing a quantization characteristic when a block <b>13</b> in the subframe <b>12</b> is encoded.

0070Incidentally, encoded information <b>20</b> in the block layer <b>13</b>S which is the lowest hierarchy in <figref idref="DRAWINGS">FIG. 6</figref> is generated in an encoding method of motion detection, prediction, orthogonal transform, quantization, variable length coding, etc., whose coding bit number are not fixed.

0071Now referring to <figref idref="DRAWINGS">FIG. 7</figref>, a decoding method according to this embodiment will be now described. First, an FSC <b>15</b> which is an identifier of a frame layer <b>11</b>S is found out from a bit stream <b>14</b> to establish synchronization of the frame layer <b>11</b>S.

0072After the establishing of synchronization of the frame layer <b>11</b>S, an SFSC <b>16</b> which is an identifier of a subframe layer <b>12</b>S is found out from the following bit stream <b>14</b> to establish synchronization of the subframe layer <b>12</b>S. Then a subframe time number SFNT <b>17</b> and a subframe space number SFNS <b>18</b> following the SFSC <b>16</b> are examined. Next, a bit stream <b>14</b> of a block layer <b>13</b>S is decoded. A method for decoding this block layer <b>13</b>S is a procedure to regenerate the block in processes of, for example, variable length decoding, reverse quantization, reverse orthogonal transform, motion compensation, etc. Finally, the subframe <b>12</b> which is a set of the regenerated blocks <b>13</b> is placed in time and space positions instructed by the SFNT <b>17</b> and the SFNS <b>18</b>. If synchronization of the decoding is lost due to a lack of a part of the bit stream <b>14</b> or an error therein, a seek for the SFSC <b>16</b> which is an identifier of the subframe layer <b>12</b>S is started. A lack or an errorof a portion of, or an error in, the bit stream <b>14</b> can be detected from, for example, that a decoded value exceeds a range specified in advance or an unexpected code word appears when the bit stream <b>14</b> is decoded. When the SFSC <b>16</b> is detected and synchronization of the subframe layer <b>12</b>S is established, the SFNT <b>17</b> and the SNFS <b>18</b> are examined as stated above, the block layer <b>13</b>S is decoded and regenerated, and the subframe <b>12</b> which is a set of the regenerated blocks <b>13</b> is placed in time and space positions instructed by the SFNT <b>17</b> and the SFNS <b>18</b>.

0073According to the first embodiment of this invention, if a part of the bit stream <b>14</b>lacksis lacking or an error occurs in the bit stream <b>14</b>, synchronization of the decoding is lost and the decoding becomes unfeasible, but correct decoding becomes possible immediately after a subframe <b>12</b> in trouble.

0074As having been described the above first embodiment by way of an example, it is alternatively possible that the frame <b>11</b>, the subframe <b>12</b> and the block are in different sizes and shapes. A bit length of each encoded information may be different from that of the above encoded information, or the frame layer <b>19</b> may be omitted, in addition.

0075According to a second embodiment of this invention, description will be now made of a method for encoding digital moving picture signals which can suppress degradation of a reproduced picture to a small extent if a subframe including a picture moving in relation to time cannot be decoded. Incidentally, it is possible here to employ a decoding method similar to that of the first embodiment.

0076In the encoding method of this embodiment, one frame <b>11</b> of digital moving picture signals is composed of, for example, 352×288 pixels. The frame <b>11</b> is divided into blocks each composed of 16×16 pixels. In other words, one frame <b>11</b> is composed of 22 blocks×18 block lines <b>21</b>. The block line <b>21</b> corresponds to the subframe <b>12</b> mentioned above.

0077In the encoding method of this embodiment, each block <b>13</b>′ is encoded from the uppermost block line <b>21</b>, as shown in <figref idref="DRAWINGS">FIG. 8</figref>, to generate encoded information. The encoded information of each block <b>13</b>′ is generated in an encoding method of, for example, motion detection, prediction, orthogonal transform, quantization, variable length coding, etc., the number of coding bits of which is not fixed. More specifically, the number of coding bits of a block <b>13</b>′ which is difficult to be encoded is large, whereas the number of coding bits of a block <b>13</b>′ which is easy to be encoded is small. In the encoding method of this embodiment, a set of blocks <b>13</b> or <b>13</b>′composes a subframe <b>12</b> (or a block line <b>21</b>) which is a unit of encoding, but the number of blocks <b>13</b> or <b>13</b>′ included in one subframe <b>12</b> or <b>12</b>′ is not fixed.

0078A manner of generating a bit stream <b>14</b> in the encoding method of this embodiment and a structure of a subframe layer <b>12</b>S will be now described with reference to FIG. <b>6</b>. When one frame is encoded, an identifier of a frame layer is encoded, and an FSC <b>15</b> is placed in a bit stream <b>14</b>. Next, the identifier of the subframe layer <b>12</b>S, a time number and a space number of that subframe, and a quantization characteristic of that subframe are encoded together, and code words of an SFSC <b>16</b>, and SFNT <b>17</b>, an SFNS <b>18</b> and an SFQUANT <b>19</b> are placed in the bit stream <b>14</b>. At the same time, block coding bit number integrated value B-add is set to zero. Following that, a block <b>13</b> is encoded and encoded information of the block <b>13</b> composed of variable codes is placed in the bit stream <b>14</b>. Concurrently, the coding bit number B of this block <b>13</b> is added to B-add. Namely, an equation, B-add=B-add+B, is computed. Similarly, blocks <b>13</b> are encoded successively, encoded information <b>20</b> of each block <b>13</b> is placed in the bit stream <b>14</b>, and a calculation of B-add=B-add+B is repeated each time. If the B-add exceeds a subframe interval SFd when encoding of a certain block <b>12</b> is completed, an identifier of the subframe, a time number and a space number of that subframe and a quantization characteristic of that subframe are encoded, and code words of an SFSC <b>16</b>, an SFNT <b>17</b>, an SFNS and an SFQUANT <b>19</b> are placed in the bit stream <b>14</b>. At the same time, a block coding bit integrated value B-add is set to zero. In other words, a new subframe layer <b>12</b>S is started to be formed from that point.

0079The subframe interval SFd is set to, for example, 540 bits. Therefore, if one frame is encoded with, for example, 6400 bits in the encoding method of this embodiment, 12 subframes <b>12</b> exist in one frame since 6400/540=11.85.

0080In the encoding method according to this embodiment, the number of blocks <b>13</b> included in a subframe <b>12</b> is varied according to a quantity of generated information of the blocks indicated in one subframe, thereby varying a spatial size of the subframe <b>12</b>, as stated above. More specifically, a subframe <b>12</b> including a block which is difficult to be encoded becomes small, whereas a subframe <b>12</b> including a block <b>13</b> which is easy to be encoded becomes large. <figref idref="DRAWINGS">FIG. 9</figref> shows an example of a structure of subframes formed in the encoding method of this embodiment.

0081According to th ethe second embodiment of this invention, a subframe <b>12</b> including a block <b>13</b> which contains a motion in relation to time, and is thus difficult to be coded is made smaller in relation to space. If such subframe <b>12</b> cannot be decoded, it is possible to suppress degradation of a quality of a reproduced picture to a small extent. In a region within a frame in which no motion in relation to time exists and degradation of the picture quality is hardly detected even if the decoding is unfeasible, a size of one subframe is large in relation to space, which allows a less volume of side information such as the subframe identifier SFSC <b>16</b>, subframe number SFNT <b>16</b> and subframe number SFNT <b>17</b>. This can prevent an encoding efficiency from being lowered.

0082As having been described the second embodiment by way of an example, it is alternatively possible that the frame <b>11</b>, subframe <b>12</b> and the block <b>13</b> are in different sizes and shapes. It is also possible to employ values of a quantity of codes of one frame and a subframe interval SFD different from those employed in the above example.

0083As obvious from the above embodiments, this invention enables correct decoding of each subframe <b>12</b> as a unit in relation to time even if a part of the bit stream <b>14</b> is missing or an error occurs therein.

0084Further, according to this invention, it is possible to suppress degradation of a quality of the reproduced picture to a small extent if a subframe <b>13</b> including a block which is in motion in relation to time cannot be decoded.

0085Still further, in a region within a frame in which no motion in relation to time exists and degradation of a quality of the reproduced picture quality is hardly detected even if the decoding is unfeasible, side information of the region is allowed to be in a small volume so that it is possible to prevent an encoding efficiency from being lowered.